1. Field of the Invention
The present invention relates to removing contaminants from an aquaculture system, and more particularly, to a recirculating marine aquaculture system and methods for using same for promoting denitrification through the process of dissimilatory sulfate reduction.
2. Description of the Related Art
The development of technology that enables the aquaculture industry to expand production of an aquatic species in urban recirculating systems requires systematic examination of each aspect of said aquatic systems. Optimal growth of the aquatic species is directly related to the environmental parameters, and as such, pollutants and waste by-products must be removed from the system to assure the species viability. Nitrogenous wastes are eliminated through the action of nitrifying and denitrifying biofilter units that rely on oxygen and reduced organic compounds, respectively, for their activity. Organic wastes are typically removed mechanically to avoid their consumption by bacteria, which uses oxygen and results in the buildup of toxic ammonia and noxious gases, such as hydrogen sulfide.
In most closed and quasi-closed aquaculture systems, ammonia is oxidized to nitrite NO2− in an aerobic biofilter by autotrophic bacteria. Nitrite is more toxic then the ammonium ion, so a second bacterium is required to oxidize the nitrite to nitrate. While nitrate is considerably less toxic than ammonium or nitrite, it can also be a problem. Nitrate is typically removed from recirculating culture systems by water exchange. However, water exchange has several drawbacks. First, water removal in aquaculture systems normally involves a slow exchange with a thorough mixing of old and new water to avoid stressing the cultured aquatic species. Second, in systems where natural sea water is unavailable, deionized water and sea salts must be mixed, which may incur heavy additional costs. Finally, the high nitrate effluent must be normally discharged. However, the discharge of the high nitrate effluent is potentially a problem, since salt water cannot typically be discharged into a sewage system or a fluvial system. Further, there is a growing environmental concern about the discharge of nitrogenous waste. Discharge permits may be complex and often require very stringent pollutant limits. Thus, attempts have been made to denitrify the discharge.
Most recirculating systems generally utilize filtration systems that rely solely on the initial stage of nitrogen waste removal, i.e. nitrification, which eliminates ammonia but results in nitrate accumulation. However, relatively few studies have been conducted on nitrate removal from recirculating systems by biological denitrification. Otte and Rosenthal (1979) used an activated sludge tank fed from the bottom with pond water and stirred with a propeller to induce denitrification using glucose and methanol as carbon and energy sources. Kaiser et al. (1989) described a similar system in a recirculating trout culture unit using hydrolyzed cornstarch as a carbon source. Van Rijn (1996) offered a novel approach for applying the denitrification process in recirculating systems without any additional support of external carbon source. By using the sludge from the fish culture as a carbon source to support the denitrification process, he demonstrated the possibility of operating a “zero discharge” system. More recent studies used methanol as a carbon source with an automated dosing system to control nitrate concentrations in the fish tanks (Lee et al., 2000).
In order to achieve anaerobic conditions that would stimulate denitrification, many filtration systems require the addition of organic compounds to promote oxygen consumption during degradation by heterotrophic bacteria (and induce anaerobic pockets), as well as to serve as electron donors to support biological nitrate reduction in denitrifying biofilters. Additionally, alcohols, volatile fatty acids and sugars, which are often used as carbon sources for such systems, often lead to bacterial blooms, toxic by-products, among other problems, and result in elevated system costs. Thus, the disadvantages of stimulating denitrifying activity by heterotrophic bacteria far often outweigh the advantages.
Accordingly, it would be advantageous to develop a system and method that uses denitrification as a viable means of nitrate removal, however, uses an alternative electron donor source that reduces or eliminates the need for external organic source material and minimizes sludge production.